By way of background, particles can be manipulated by subjecting them to travelling electric fields. Such travelling fields are produced by applying appropriate voltages to microelectrode arrays of suitable design. The microelectrodes have the geometrical form of parallel bars, which may be interrupted by spaces to form channels, as shown in FIG. 1 and may be fabricated using standard metal sputtering and photolithographic techniques as described by Price, Burt and Pethig, Biochemica et Biophysica, Vol. 964, pp. 221-230. Travelling electric fields are generated by applying voltages of suitable frequency and phases to the electrodes as described in a paper, title "Separation of small particles suspended in liquid by nonuniform travelling field", by Masuda, Washizu and Iwadare, IEEE Transactions on Industry Applications, Vol. IA-23, pp. 474-480. Masuda and his coworkers describe how a series of parallel electrodes (with no channels) supporting a travelling electric field can, in principle, be used to separate particles according to their electrical charge and size (weight). Masuda et al have not however described a practical demonstration of such a particle separation method.
In a paper entitled "Travelling-wave dielectrophoresis of microparticles" by Hagedorn, Fuhr, Muller and Gimsa (Electrophoresis, Vol. 13, pp. 49-54) a method is shown for moving dielectric particles, like living cells and artificial objects of microscopic dimensions, over microelectrode structures and in channels bounded by the electrodes. The travelling field was generated by applying voltages of the same frequency to each electrode, with a 90.degree. phase shift between neighbouring electrodes.
In "Electrokinetic behaviour of colloidal particles in travelling electric fields: Studies using Yeast cells" by Y Huang, X-B Wang and R Pethig J. Phys. D. Appl. Phys. 26 1993 1528-1535, an analysis supported by experiment is made of the "travelling-wave dielectrophoresis" (TWD) effect described by Hagedorn et al (paper cited above). The phenomenological equation ##EQU1## is developed by Huang et al, to show that the TWD velocity is a function of the square of the particle radius (r), the square of the electric field strength (A(0)), the periodic length of the travelling field (.lambda.), medium viscosity (.eta.) and the imaginary part of the Clausius-Mossotti factor f (.epsilon..sub.p,.epsilon..sub.m) defining the dielectric properties of the particle and the suspending medium in terms of their respective complex permittivities .epsilon..sub.p and .epsilon..sub.m. This equation provides, for the first time, a practical guide for the design of travelling wave electrode systems for the manipulation and separation of particles.
The movement of particles by high frequency fields using electrode arrays is also described in WO092/07657.
Although the phenomenon in question is usually termed "travelling wave dielectrophoresis", we have now demonstrated that this is something of a misnomer as the force which acts on the particles to produce translational movement is not the dielectrophoresis force but rather that which acts in electrorotation. This force is related to the imaginary component of the polarizability of the particle within its surrounding medium. However, as is discussed in more detail below, particle migration only occurs for travelling wave frequencies which produce negative dielectrophoretic forces on the particle. (Dielectrophoretic forces are related to the real component of the polarizability of the particle within its surrounding medium.) These forces are responsible for lifting the particle away from the electrodes and the channel between the electrodes. We accordingly prefer to refer to the phenomenon called previously "travelling wave dielectrophoresis" by the name "travelling wave field migration" (TWFM). We have established that to obtain TWFM, two separate criteria have to be met. First, a frequency must be selected at which the dielectrophoresis force acting on the particles is negative, i.e., such as to repel the particles from the electrodes. This, we have found, requires the real component of the dipole moment induced in the particles to be negative.
Second, the frequency selected has to be such that the imaginary component of the dipole moment induced in the particles is non-zero (whether positive or negative) to produce a force displacing the particles along the array of electrodes.